chore(receiver): packaging, k8s manifests, CI/release, docs

Dockerfile.receiver (CGO-free, /data volume), receiver image in CI and
tag releases, receiver binary in release archives, make build-receiver,
example k8s manifests (deployment/service/ingress/secret/pvc) under
deploy/k8s/receiver/, and README coverage including sender/receiver
token pairing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Dockerfile.receiver

@@ -0,0 +1,26 @@
+# Build stage
+FROM golang:1.25-alpine AS builder
+
+WORKDIR /src
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+ARG VERSION=dev
+RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags="-s -w -X main.version=${VERSION}" -o /out/exploredns-receiver ./cmd/exploredns-receiver
+
+# Final stage
+FROM alpine:3.21
+
+RUN apk --no-cache add ca-certificates
+
+COPY --from=builder /out/exploredns-receiver /usr/local/bin/exploredns-receiver
+
+ENV RECEIVER_SQLITE_PATH=/data/exploredns-receiver.db
+VOLUME /data
+
+EXPOSE 8080
+
+ENTRYPOINT ["exploredns-receiver"]